Scattering

Scattering happens when light waves interact with particles or irregularities in a medium, causing the light to change direction.

  • When light encounters obstacles, such as molecules in the atmosphere or imperfections in a surface, it can undergo various processes.
  • These include reflection, where the light bounces back like a mirror, as well as refraction, diffraction, and absorption, where the light is bent, spread out, or absorbed by the material.
  • Scattering plays a  role in various natural phenomena, such as the colour of the sky, the appearance of clouds, and the shimmering of water surfaces.

Scattering does not take place:

  • When parallel rays of light reflect off a smooth, flat surface like a mirror, producing a distortion-free reflection.
  • When parallel rays of light reflect off a smooth convex surface (although the reflection appears magnified).
  • When parallel rays of light reflect off a smooth concave surface (although the reflection typically appears smaller and inverted).
  • When parallel rays of light pass through translucent materials containing dissolved substances like dyes.
About regular scattering
  • Regular scattering happens when light bounces off a smooth, curved surface in a predictable way, creating a clear and undistorted image.
  • Think about a spoon in a glass of water. The smooth, curved surface of the spoon predictably bends the light, making the spoon appear slightly bent or magnified. This is an example of regular scattering.
  • Regular scattering often occurs when parallel rays of light hit smooth, transparent objects like raindrops or prisms. In these cases, the light bends (refracts) in a predictable way depending on the angle it hits the object and the materials involved.
  • This predictable bending can sometimes separate white light into its component colours, creating a rainbow effect known as chromatic dispersion.
  • On a microscopic level, all types of scattering follow the laws of reflection and refraction (Snell’s law).
  • Let’s look at two cases of regular scattering in more detail:
    • When parallel rays of light with a single wavelength strike and enter an object like a raindrop or prism, their path depends on the initial point of impact, the refractive indices of air and water, and the object’s surface properties.
    • When parallel rays of incident light with a single wavelength meet the curved surface of a transparent medium at various points, the different angles at which they strike the surface and experience deflection mainly determine how they scatter as they exit the medium.
About random scattering
Random scattering
  • Random scattering occurs when a material, due to irregularities or imperfections on its surface, reflects or transmits light rays in various unpredictable directions.
  • This scattering can produce a variety of effects:
    • Reflected light may appear hazy or lack detail, or there may be no clear reflection at all.
    • When light passes through sheets of glass with irregular yet smooth surfaces, random scattering distorts the view of the world beyond, making the image blurry and confused.
    • A reflection that is free of the effects of random scattering is called a specular reflection. Mirrors generally produce specular reflections.
Diffuse light
  • Diffuse light is a specific type of random scattering that occurs when light bounces off rough or uneven surfaces.
  • In these cases, the light scatters in all directions, creating a soft, even glow.
  • The overall structure and composition of a material can also cause diffuse light.
  • This happens when light travels through a medium that contains foreign materials, suspended particles, or has an irregular internal structure or variations in density.
  • Translucent materials containing dissolved substances, however, typically don’t cause random scattering because the particles are too small.
  • On a microscopic scale, all objects adhere to the law of reflection; however, when surface irregularities are larger than the wavelength of light, the light undergoes scattering leading to diffusion.
About scattering in raindrops
  • Scattering in raindrops obeys the laws of both reflection and refraction, commonly referred to as Snell’s law. Here are three related descriptions of what causes scattering when visible light strikes a raindrop:
    • When light of a specific wavelength strikes the surface and enters a raindrop its subsequent path depends upon the point of impact, and the refractive indices of water and air.
    • When rays of light of a single wavelength strike a raindrop at different points, scattering is primarily determined by the angles at which they enter the droplet.
    • The interaction between refraction and chromatic dispersion gives rise to the appearance of rainbow colours when parallel white light rays strike various points on the surface of a raindrop.